This invention pertains to cardiac pacemakers and, in particular, to systems and methods for monitoring the effects of pacing and adjusting pacing parameters in accordance therewith.
Cardiac pacemakers are medical devices, usually implantable, that provide electrical stimulation in the form of pacing pulses to selected chambers of the heart (e.g., the right atrium and/or the right ventricle). Pacemakers typically have a programmable electronic controller that causes the pacing pulses to be output in response to lapsed time intervals and sensed electrical activity (i.e., intrinsic heart beats). Implantable pacemakers sense intrinsic cardiac electrical activity by means of internal electrodes disposed near the chamber to be sensed, with the depolarization waves associated with contractions of the atria and ventricles referred to as P waves and R waves, respectively. In order to cause such a contraction in the absence of intrinsic activity, a pacing pulse (referred to as an A-pace or V-pace in the case of an atrium or ventricle, respectively) with energy above a certain pacing threshold is delivered to the chamber.
Most pacemakers are programmed to operate in a so-called demand mode (a.k.a., synchronous mode), where a pacing pulse is delivered to a heart chamber during a cardiac cycle only when no intrinsic beat by the chamber is detected. An escape interval is defined for each paced chamber, which is the minimum time interval in which a beat must be detected before a pace will be delivered. The ventricular escape interval thus defines the minimum rate at which the pacemaker will allow the heart to beat, sometimes referred to as the lower rate limit. If functioning properly, the pacemaker in this manner makes up for a heart""s inability to pace itself at an appropriate rhythm.
In order for a pacemaker to control the heart rate in the manner described above, the paces delivered by the device must achieve xe2x80x9ccapture,xe2x80x9d which refers to causing sufficient depolarization of the myocardium that a propagating wave of excitation and contraction result (i.e., a heart beat). A pacing pulse that does not capture the heart is thus an ineffective pulse. This not only wastes energy from the limited energy resources (battery) of pacemaker, but can have deleterious physiological effects as well, since a demand pacemaker that is not achieving capture is not performing its function in enforcing a minimum heart rate. A number of factors can determine whether a given pacing pulse will achieve capture, but the principal factor of concern here is the energy of the pulse, which is a function of the pulse""s amplitude and duration or width. Programnmable pacemakers enable the amplitude and pulse width of pacing pulses to be adjusted, along with other parameters. It is therefore desirable to perform a capture verification test at selected times in order to ascertain whether capture is being achieved by a pacemaker so that such parameters can be adjusted if needed.
A common technique used to determine if capture is present during a given cardiac cycle is to look for an xe2x80x9cevoked responsexe2x80x9d immediately following a pacing pulse. The evoked response is the wave of depolarization that results from the pacing pulse and evidences that the paced chamber has responded appropriately and contracted. By detecting the evoked P-wave or evoked R-wave, the pacemaker is able to detect whether the pacing pulse (A-pulse or V-pulse) was effective in capturing the heart, that is, causing a contraction in the respective heart chamber. Capture verification can be performed in the clinical setting, with the clinician then adjusting pacing parameters so that the heart is reliably paced. It is desirable, however, for the pacemaker itself to be capable of verifying capture so that loss of capture can be detected when it occurs with pacing parameters then adjusted automatically, a function known as autocapture. An autocapture function provides the pacemaker with extended longevity, greater ease of use, and greater patient safety. In order for a pacemaker to detect whether an evoked P-wave or an evoked R-wave occurs immediately following an A-pulse or a V-pulse, a period of time, referred to as the atrial capture detection window or the ventricular capture detection window, respectively, starts after the generation of the pulse. Sensing channels are normally rendered refractory (i.e., insensitive) for a specified time period immediately following a pace in order to prevent the pacemaker from mistaking a pacing pulse or afterpotential for an intrinsic beat. This is done by the pacemaker controller ignoring sensed events during the refractory intervals, which are defined for both atrial and ventricular sensing channels and with respect to both atrial and ventricular pacing events. Furthermore, a separate period that overlaps the early part of a refractory interval is also defined, called a blanking interval during which the sense amplifiers are blocked from receiving input in order to prevent their saturation during a pacing pulse. If the same sensing channels are used for both sensing intrinsic activity and evoked responses, the capture detection window must therefore be defined as a period that supercedes the normal refractory period so that the sensing circuitry within the pacemaker becomes sensitive to an evoked P-wave or R-wave.
In certain devices, capture verification is performed by delivering a pacing pulse and attempting to sense an evoked response using the same electrode. Such a technique suffers from a number of problems, however. One is the induced polarization that builds up on an electrode after a pacing pulse which interferes with sensing by the electrode. Another is that an evoked response is a wave of depolarization that necessarily moves away from a pacing electrode responsible for the depolarization. Since such a wave of depolarization causes an electric field equivalent to a moving dipole disk, the potential resulting from that field is best sensed by an electrode that lies in the path of the wave. Also, if a backup pacing pulse is to be delivered, using the same pulse generator that produced the non-capturing pacing pulse for this purpose means that the output capacitor of the pulse generator must be recharged before another pace can be delivered, which takes some time.
The present invention is an apparatus and method for verifying capture by a pacing electrode in a multi-site pacemaker. Such a pacemaker includes a plurality of sensing/pacing channels, with each such channel comprising an electrode for disposing near a right or left chamber of the heart, a pulse generator for outputting pacing pulses, and a sense amplifier for detecting sense signals. A controller controls the operation of the pulse generators in response to sensed events and lapsed time intervals and in accordance with a programmed pacing mode. The controller is programmed to test a selected sensing/channel for presence or loss of capture by performing a capture verification test at a selected time. A capture verification test on a selected pacing electrode is performed by sensing whether an evoked response occurs during a capture detection window period following the output of a pacing pulse through the test electrode. The programming may dictate that a capture verification test is performed at periodic intervals or in response to a command received via a telemetry interface from an external programmer.
In accordance with the invention, a dedicated evoked response sensing channel is provided which includes a sense amplifier for sensing an evoked response generated after a pacing pulse. Also, a switching circuit is provided that switches the input of the evoked response sensing channel to a selected electrode of the sensing/pacing channels before the capture verification test is performed. Preferably, the input of the evoked response sensing channel is switched to an electrode of a sensing/pacing channel other than the channel being tested during a capture verification test. The sense amplifier of the evoked response sensing channel is then blanked during the capture verification test for a specified blanking period following a pacing pulse output by the tested sensing/pacing channel, and the blanking period is followed by a capture detection window during which an evoked response may be sensed.
Another advantage of using a dedicated sense amplifier for detecting evoked responses to paces is that a bandpass filter with a wide passband can be used. Sensing/pacing channels used for sensing intrinsic cardiac activity employ filters with narrow passbands in order to best detect the signals they are designed to sense. A typical signal due to an evoked response that is processed through such a narrow passband filter, however, results in a waveform that makes detection of the evoked response more difficult. Thus, the sensing amplifier of the dedicated evoked response sensing channel is preferably one with a wide passband.
Also in accordance with the invention, the controller is programmed to output a backup pacing pulse through a sensing/pacing channel if loss of capture is detected during a capture verification test. In the event no evoked response is detected by the pacemaker during the atrial or ventricular capture detection window, and if the pacemaker is operating in an autocapture mode, the pacemaker delivers a backup pacing pulse some time after the capture detection window expires. The backup pacing pulse may be an A-pulseor V-pulse of greater amplitude, pulse width or both than the initial atrial or ventricular stimulation pulse. The backup pacing pulse is designed to capture the heart when the initial, lower amplitude and/or lower pulse width stimulation pulse was unable to do so. The backup pacing pulse may be output through a sensing/pacing channel other than the channel being tested during the capture verification test. In certain embodiments, the backup pacing channel uses the same electrode as the evoked response sensing channel. In a pacemaker that provides multi-site pacing, such as a biventricular pacemaker, normal pacing is preferably suspended in the sensing/pacing channel used to sense the evoked response and provide backup pacing.